Means for merging two or more video signals to provide a single composite video signal is known in the art. An example of such video merging is presentation of weather-forecasts on television, where a weather-forecaster in the foreground is superimposed on a weather-map in the background.
Such prior-art means normally use a color-key merging technology in which the required foreground scene is recorded using a colored background (usually blue or green). The required background scene is also recorded. In its simplest form, the color-key video merging technique uses the color of each point in the foreground scene to automatically “hard” switch (i.e., binary switch) between the foreground and background video signal. The color-key video merging technique uses the color of each point in the foreground scene to automatically switch between the foreground and background video signal. In particular, if a blue pixel is detected in the foreground scene (assuming blue is the color key), then a video switch will direct the video signal from the background scene to the output scene at that point. If a blue pixel is not detected in the foreground scene, then the video switch will direct the video from the foreground scene to the output scene at that point. After all points have been processed in this way, the result is an output scene which is a combination of the input foreground and background scenes.
In more complex forms of the color-key video merging technique, the effects of switching may be hidden and more natural merging may be achieved. For instance, shadows of foreground subjects may be made to appear in the background.
The color-key merging technique is simple, and cheap hardware for this method has been available for some time. As a result, color-key insertion can be performed on both recorded and live video. It is used widely in live television for such purposes as superimposing sports results or images of reporters on top of background scenes, and in the film industry for such purposes as superimposing foreground objects (like space-ships) onto background scenes (like space-scenes).
However, there are two important limitations of color-key merging technology. First, this technique cannot be used to combine video sources where the separation color (e.g., blue or green) in the scene cannot be controlled by the employer of this technology. This has often limited the use of color-key insertion to image sequences recorded in a broadcasting or film studio. Second, it is not currently possible to automatically combine video signals in such a way that patterns inserted from one sequence follow the motion of objects (foreground or background) in the other sequence so that the inserted patterns appear to be part of these objects. While, in the past, synchronization of the motions of background and foreground scenes has been performed manually in a very limited number of film productions, such manual synchronization is highly expensive and tedious and requires that the video material be prerecorded and not ‘live’.